Re: ext4: first write to large ext3 filesystem takes 96 seconds

On Mon, Jul 07, 2014 at 05:13:49PM -0400, Benjamin LaHaise wrote:s

Hi folks,

I've just ran into a bug with the ext4 codebase in 3.4.91 that doesn't seem 
to exist in ext3, and was wondering if anyone has encountered this before.  
I have a 7.4TB ext3 filesystem that has been filled with 1.8TB of data.  
When this filesystem is freshly mounted, the first write to the filesystem 
takes a whopping 96 seconds to complete, during which time the system is 
reading about 1000 blocks per second.  Subsequent writes are much quicker.  
The problem seems to be that ext4 is loading all of the bitmaps on the 
filesystem before the first write proceeds.  The backtrace looks roughly as 
follows:

So the issue is that ext3 will just allocate the first free block it
can find, even if it is a single free block in block group #1001,
followed by a single free block in block group #2002.  Ext4 tries a
harder to find contiguous blocks.

If you are using an ext3 file system format, the block allocation
bitmaps are scattered across the entire file system, so we end up
doing a lot random 4k seeks.

We can try to be a bit smarter about how we try to search the file
system for free blocks.

Out of curiosity, can you send me a copy of the contents of:

/proc/fs/ext4/dm-XX/mb_groups

Thanks!!

					- Ted